1. Field of the Invention
This invention relates to improvements in electromagnetic railgun construction which substantially improve railgun performance and durability, and facilitate rebuilding between successive firings. In particular, the invention relates to a railgun having a containment structure for preloading bore seals, an internal geometry which prevents insulator intrusion into the bore and promotes sealing of the bore, a flash tube to commutate precursor current back into the gun breech, and a projectile geometry providing a higher velocity without sacrificing in-bore projectile stability.
2. Description of the Related Art
Thermodynamic guns are widely used and generally understood in a broad context. In an ordinary thermodynamic gun, a propellant burns to generate high pressure gas that pushes a projectile down a bore. While thermodynamic guns are used in many applications besides weapons--for example scientific and industrial applications--their use is somewhat limited because of the maximum velocities attainable. Thus, physical limitations limit the projectile from such thermodynamic guns from reaching velocities much greater than one kilometer per second.
Electromagnetic railguns have been widely investigated since World War II as an alternative to thermodynamic guns because of the possibilities of achieving projectile hypervelocities (greater than one kilometer per second). Hypervelocity guns and launchers are under development for a wide range of applications, including anti-missile systems for strategic defense, impact fusion for nuclear energy production, and launching systems for satellites and spacecraft.
The early electromagnetic railguns incorporated a solid armature which was propelled between the rails By the electromagnetic force generated by the current flow through the armature and the rails. However, it was soon found that at high speeds around one kilometer per second, the rails and armature were substantially damaged, possibly as a result of ohmic heating and/or internal forces. Further, increases in current flow tended to only increase rail and armature gouging without an increase in armature velocity. Thus, armature velocities in excess of one kilometer per second were not practically attainable for railguns using solid armatures.
In the early 1970's, R. A. Marshall, J. P. Barber, and others at the Australian National University, Canberra, Australia, developed railguns using plasma armatures which could obtain hypervelocities and could make efficient use of high current, pulsed power supplies, such as compulsators and homopolar generators. Plasma armature railguns are generally described in S. C. Rashleigh and R. A. Marshall, "Electromagnetic Acceleration of Macroparticles to High Velocities," 49 J. App. Phys. 2540 (Apr. 1978) (incorporated herein by reference). Such power supplies are broadly illustrated in U.S. Pat. Nos. 4,200,831; 4,459,504; 4,246,507; and U.S. patent application Ser. No. 067,410 filed June 24, 1987 (incorporated herein by reference).
In recent years, additional research has revealed numerous problems associated with very high current plasma armatures. For example, at the high currents necessary to obtain hypervelocities, rail erosion has been a significant problem which essentially relegates the railgun to a one or two shot application. Additionally, plasma armature type railguns require a sealed bore (open only at the muzzle) capable of withstanding the substantial electromagnetic forces generated; the gaskets, seals, and insulator material associated with such bores have proven a significant problem. For example, in addition to rail and insulator damage, metallic deposits often adhere to the insulator surfaces after firing, causing arcing problems during subsequent firing.
In addition to the practical difficulties of plasma-type electromagnetic railguns, several fundamental problems have become of increasing concern. For example, at the typical working currents in question, the dissipative armature voltage drop is an order of magnitude greater than desirable. Perhaps more fundamentally, plasma armatures are designed to accelerate a projectile using base pressures. Base pressure acceleration (such as also used in thermodynamic guns) places severe design limits on the projectile. For example, a projectile must be able to withstand the extreme temperature and pressures exerted at its base by the driving plasma.
At present, railgun performance is limited to a laboratory environment by the lengthy rebuild periods between successive firings and the inability of the bore materials to withstand the extreme forces during firing. Similar considerations dictate the feasibility and economy of railgun applications outside of the laboratory where the railgun will be operated under somewhat less stringent conditions. Therefore, it is desirable to provide a more durable railgun construction to increase the number of shots between rebuilding times and to facilitate the rebuilding process.